Well, that’s not so great.

You might have heard that Apple recently refreshed the MacBook Pros with the new 8th-generation Intel Coffee Lake CPUs, including the top-of-the-line Core i9 8950HK. And you may have also heard that the very same configuration generates so much heat to the point where the MacBook’s chassis and cooling system could simply not handle and resulted in the CPU being unable to maintain base clocks. Obviously, this is quite a big issue and it really begs the question of why Apple decided to offer a $300 option for a better CPU when the chassis and cooling solution may not be up to snuff and can result in performance being worse than the i7 from the 8th and even 7th generation.

But why exactly is fitting such a CPU in a chassis as compact as the MacBook Pro such a risky proposition? It’s a lot to do with space and heat. Let me explain;

To do work, a CPU requires electrical energy, which it receives from the computer’s power supply unit. When it receives electrical energy, the CPU is then able to perform any potential given task, but it also generates heat as a byproduct. The amount of heat generated depends on factors such as workload, utilization, core count, clockspeed and TDP among others, though it is assumed that the higher-end the chip is, the more heat it will generate.

Heat is a form of energy, meaning it cannot be destroyed, only transferred or converted. Heat is transferred in one of 3 ways;

Conduction: Transfer through contact

Convection: Transfer of heat through motion

Radiation: Transfer through electromagnetic rays

We’ll be focusing particularly on radiation for this one.

Most computers utilize active cooling, which utilizes fans and heatsinks alongside heatpipes (higher end solutions use liquid or vapor chambers) to whisk heat away from the processor and uses thermal compound between the cooler and processor to improve conductivity. Some laptops also utilize passive cooling, which uses the chassis to aid in cooling. The MacBook Pro for instance, utilizes a mix of both.

However, herein lies the problem with the MacBook Pro with Core i9 alongside other similarly equipped notebooks with a similar form factor like the refreshed Dell XPS15; space.

While these laptops utilize active cooling, not all of that heat is conducted and whisked away by the cooling system. Some amount will be radiated throughout the chassis. Not much of an issue on big desktop replacements and actual desktops due to their large internal volume but in thin machines, there's far less space to radiate that heat, which is why some machines like the aforementioned MacBook Pro and Razer Blade Pro w/ GTX 1080 have very high surface temperatures. This also translates to higher internal temperatures which increases the potential for the CPU to throttle.

A double whammy for the MacBook Pro and others like the XPS15 is the choice of cooling system. To recap, the Core I9 8950HK is a high-end 6C 12T CPU which has a 2.9GHz base clock but goes up to 4.8GHz when turbo boosted on single-treaded tasks if power and temperature limits allow. The CPU has a 45W TDP but is also known to consume up to 150W of power when in its boosted state. Obviously, that's a very power-hungry CPU and that also translates into a lot of heat, hence why the CPU is usually seen in big, bulky gaming laptops (and many of those struggle to maintain turbo but can maintain base). It's not uncommon for these laptops alongside some 8750H laptops to gain upgraded cooling systems to aid in whisking heat away, such as the refreshed ASUS RoG STRIX GL line which has a significantly upgraded cooling system to accommodate the beefier CPU.

The issue with the MacBook and XPS is that while the cooling system is (barely) adequate to handle a Kaby Lake 4C 8T Core i7 CPU without throttling below base, the cooling solution itself does not appear to have been upgraded for the 6C 12T Coffee Lake parts, especially for the 8950HK. I'm not going to make assumptions here since I probably don't know as much as the engineers who did these, but conventional wisdom would tell me that to handle a CPU that is much more powerful and generates more heat as a consequence, the cooling system should be upgraded to better handle the increased thermal load. Sadly, that doesn't appear to be the case for both the MacBook and XPS especially since the latter has had issues with VRM throttling. It's also worth noting that despite the throttling, the XPS's cooling system is better able to handle the heat load, although the VRM temperatures are still a bit of a concern.

There is one outlier that we haven't talked about yet; liquid metal. In some laptops, applying liquid metal thermal paste can significantly improve temperatures to the point where throttling disappears. The reason why I haven't yet mentioned it is because these laptops are very new and haven't have had LM repasted. There is a chance that LM can improve thermals, but not much info is out there.

Bottom line is this. Thin machines combined with super beefy CPUs will always run into heating issues particularly due to their limited internal volume for better cooling and extra space for heat to radiate. It's one of the reasons why I've kept saying that the whole thinness race should really end if we want these super powerful CPUs to run in laptops without a lot of compromise. The MacBook Pro is already in ultra portable territory when it comes to size and that combined with a power-hungry Core i9 and a cooling system that is unable to keep the fury in check results in a machine that just runs too hot to maintain base.

I don't think an i9 should have been offered in the first place if it was found that it could have throttled to the point of being worse than a last generation product. While I'm all for increased portability, there's a balance to be had and I think Apple (and Dell plus Razer) probably tipped the portability scale a bit too far.